1. Field of the Invention
The present invention relates to an image sensor or reading device used for a facsimile, scanner, and the like. More particularly, the present invention relates to the structure to improve the resolution of an image sensor constituting a read section of an image reading device and to prevent an increase of a dark output signal.
2. Description of the Related Art
A conventional image sensor or reading device of the close contact type, which is used for a facsimile, scanner and the like, is made up of a fluorescent lamp, an image sensor with a width equal to the width of an original document, and a 100% magnification optical system for imaging the reflected light from the document on the image sensor. Reflected light representing the optical densities of images on the original document is converted by a linear array of light receiving elements in the image sensor to corresponding electrical signals. The electrical signals are sequentially outputted in the form of image signals corresponding to one scan line (main scan direction) of the original document.
An image reading device using the 100% magnification system may be made more compact than an image reading device using a reduction optical system. In the former image reading device, a rod lens array may be used for the 100% magnification optical system. However, use of the rod lens array limits the degree of size reduction of the image reading device.
To cope with this, an extremely small image reading device has been proposed in which EL light emitting elements and an image sensor of the close contact type are fabricated into a unit.
This type of the image read device will be described with reference to FIG. 1. As shown, the image reading device is structured such that light receiving elements 100 are formed on a substrate 10 in opposition to EL light emitting elements 200 formed on a substrate 20, e.g., glass. A transparent layer 30 is interposed therebetween. The light receiving elements 100 include picture elements (pixels) 101 as optoelectric transducing elements, which are linearly arrayed.
Light emitted from the EL light emitting elements 200 illuminates the surface of an original 400, e.g., a document, that is disposed on the top surface of the substrate 20, which is opposite to the surface in contact with the light emitting elements. The light 500 reflected by the document surface is incident on the pixels 101 of the light receiving elements 100.
In the image reading device, the substrate 20 is made of the same material (glass) as that of the transparent layer 30. Accordingly, the reflectivity of the substrate is equal to that of the transparent layer, which creates the following problems.
Consider a case that the reflected light from the document 400 is incident on pixel 101a of the light receiving elements 100, and the reflected light beams 500 and 600 hit the pixel 101a. The reflected light beam 500 comes from an area A on the document that is to be read by the pixel 101a, but the reflected light beams 600 comes from an area outside the area A. The pixel 101a stores a quantity of charge amounting to a total amount of the reflected light from the area A and the reflected light 600 from outside of the area A. Accordingly, the pixel 101a produces an electrical signal containing unnecessary pictorial information. This reduces the resolution, i.e., modulation transfer function (MTF).
With reference to FIG. 2, in the light-source contained image sensor thus structured, refractive index n.sub.2 (about 1.5) of the glass substrate 20, refractive index n.sub.3 (about 1.4) of the general adhesive layer 30 and refractive index n.sub.1 of gas satisfy the following relation; n.sub.2 .ltoreq.n.sub.3 &gt;.perspectiveto.1.0. Air must be present between the transparent substrate 20 and the surface of the original (since the original imperfectly contacts the transparent substrate 20). Of the light beams emitted from the EL light emitting element 200, the light beams emitted at angles larger than an angle .theta..sub.1 as given by the following relation, are totally reflected on the surface of the transparent substrate 20. EQU .theta..sub.1 =sin.sup.-1 (n.sub.1 /N.sub.2).
Of the light beams totally reflected, the light beams reflected at angles smaller than angle .theta..sub.2 as given by the following relation, are not totally reflected at the boundary, but enter the light receiving elements 100 through light incident windows 26 formed in the EL light emitting elements 200. EQU .theta..sub.2 =sin .sup.-1 (n.sub.3 /n.sub.2).
The totally reflected light beams are present regardless of the presence or absence of the original document. Accordingly, by merely turning on the EL light emitting elements 200, part of the emitted EL light will be reflected to be incident on the light receiving elements 100 to cause flare. The dark output signal of the light receiving element 100 becomes larger than ground, and the dynamic range of the light receiving elements 100 becomes more narrow. This is disadvantageous for image reading in multiple gray levels.
With reference to FIG. 3, light receiving elements 100, which are formed on a substrate 10 made of any of glass, ceramic and the like, and EL light emitting elements 200, which are formed on an EL substrate 20 made of transparent material, for example, glass, are bonded by transparent insulating adhesive (adhesive layer 30). The structure extends horizontally as viewed in the drawing (main scan direction).
The light receiving elements 100 include individual electrodes 121, which are made of chromium (Cr), for example, and are discretely arrayed horizontally in FIG. 3 (main scan direction), a strip like optoelectric transducing layer 122 made of amorphous silicon (a - Si), and a strip like transparent electrode 123 made of ITO.
The EL light emitting elements 200 include a transparent electrode 241 made of any of ITO, In.sub.2 O.sub.3, SnO.sub.2, and the like, an insulating layer 242 made of any of Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, BaTiO.sub.3, and the like, a light emitting layer 243 made of ZnS : Mn, for example, another insulating layer 242 made of the same material as that of the other layer 242, and opaque electrodes 244 made of metal, e.g., aluminum, which are layered in successive order. In the EL light emitting elements thus structured, when a voltage is applied between the transparent electrode 241 and the opaque electrodes 244, the light emitting layer 243 sandwiched by them emits light, which in turn illuminates an original document 400 through the transparent electrode 241. The light emitted from the light emitting layer 243 is emitted from the obverse side of the transparent electrode 241.
Rectangular light emissive windows 245 are formed in the opaque electrode 244. The light emitted from the light emitting layer 243 is reflected by the original document 400, and the reflected light passes through the windows 245 and is incident on the light receiving portions of the light receiving elements 100 (see Japanese Patent Unexamined Publication No. 59-210664).
The image reading device shown in FIG. 3 has the following problems. Light "p" emitted from the light emitting layer 243 of the EL light emitting elements 200 is emitted from the obverse side of the transparent electrode 241. The light illuminates the original document 400, and is reflected by the original document. The reflected light passes through the windows 245 and is incident on the light receiving portions of the light receiving elements 100. In the device, light "q" emitted from the portions of the light emitting layer 243 along the periphery of the windows 245 sometimes passes through the windows 245 and enters the light receiving portions of the light receiving elements 100 directly. It is desirable to receive only the reflected light portion of the emitted light "p." If the direct incident light "q" from the light emitting layer 243 is additionally received, the dark output signal of the image reading device is increased.
Some part of the light "r," which is emitted from the light emitting layer 243, is emitted from the obverse side of the transparent electrode 241, and is not directed from the EL substrate 20 toward the original document 400. Instead, the light is totally reflected on the surface of the EL substrate 20. The totally reflected light r' (part of the emitted light "r" as totally reflected) passes through the windows 245 and enters the light receiving portion of the light receiving elements 100 associated with the window. The totally reflected light r, also increases the dark output signal of the image reading device.
Also, light emitted from the EL light emitting elements is emitted from the obverse side of the transparent electrode 241. When the emitted light "p" passes from the EL substrate 20 toward the original document 400, some part of the emitted light "p" is totally reflected on the surface of the EL substrate 20 due to the difference between the refractive index of the glass of the EL substrate 20 and air present between the original document 400 and the substrate 20. The totally reflected light "s" passes through each window 245 and the light receiving portion of the corresponding light receiving elements 100. The totally reflected light "s" further increases the dark output signal of the image reading device.